This invention relates generally to computed tomograph (CT) imaging and, more particularly, to a calibration of a CT system.
In at least some computed tomograph (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a post patient collimator for collimating scattered x-ray beams received at the detector. A scintillator is located adjacent the post patient collimator, and photodiodes are positioned adjacent the scintillator.
Uniformity between individual detector elements is important for securing good image quality of CT images. Otherwise, anomalies may occur in the collected data. A consequence of data anomalies are image distortions, commonly referred to as artifacts. Detector uniformity may be impacted by many factors including radiation damage and sensitivity of the scintillator materials. To correct for this uniformity, periodic calibrations of the detector are required.
A common method for calibration employs devices known as a phantoms. Different size phantoms, providing known attenuation paths for x-ray beams passing therethrough, are utilized to generate calibration data for different size objects. Using any non-uniformity demonstrated during the calibration, an error value is generated for the specific size object. The generated error value is then used to correct the measured values of the detector so that detector response is uniform for subsequent use when imaging a patient.
The calibration method, described above, has the major drawback of requiring separate calibrations for different modes of operation, or scan types, of the imaging system. These scans include different size objects and different types of filtration of the x-ray beam, using a filtration device. Such a filtration device, in one known system, is a bow-tie filter having a head portion and a body portion. As a consequence, a separate calibration must be performed for each scan type. Therefore, the calibration of the imaging system is very time consuming and is likely to cause inconvenience to the CT system operator.
Accordingly, it would be desirable to provide a CT system that reduces the number of calibration scans which must be performed. It is also desirable to provide a method for utilizing calibration data from a first scan type to determine the error value for other scan types.